<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text">
<input type="button" value="平方" onclick="f1()">
<input type="button" value="相加" onclick="f2()">
<h1>结果:<span></span></h1>
<script>
  let i = document.querySelector("input");
  let s = document.querySelector("span");



  function f1() {
    //Not a Number:不是一个数  当NaN和任何数值进行任何运算时结果都为NaN
    if (isNaN(i.value)){
      s.innerText = "输入错误!";
      return;
    }
    s.innerText = i.value*i.value;
  }
  function f2() {
    if (isNaN(i.value)){
      s.innerText = "输入错误!";
      return;
    }
    //当字符串进行-*/运算时会自动转成数值, 加法运算是拼接
    // s.innerText = parseFloat(i.value)+parseFloat(i.value);
    s.innerText = i.value*1+i.value*1;


  }
</script>
</body>
</html>